Rivastigmine is used in the treatment of Alzheimer's disease which selectively inhibits acetylcholinesterase. Its structure is as shown formula (I) below:

U.S. Pat. No. 5,602,176 and GB2409453 released for the first time the synthesis methods for rivastigmine: first, prepare racemic rivastigmine through a series of reactions, and then salify it with (+)-Di-p-toluoyl-D-tartaric acid monohydrate (D-(+)-DTTA); then it goes through five times of recrystallization to get rivastigmine with an optical purity greater than 99%, and the resolution yield is only 26.8% WO2004037771 discloses a method to resolve rivastigmine intermediates: use S-(+)-camphorsulfonic acid to resolve racemic intermediate (RS)-3-(1-(dimethylamino)ethyl)phenol, let it go through three times of recrystallization and purification to obtain optically pure(S)-3-(1-(dimethylamino)ethyl)phenol; then condense it with ethyl(methyl)carbamic chloride before salifying it with L-(+)-tartaric acid to receive rivastigmine with the total yield of 18.73%.
WO2007025481 discloses a method: use S-phenethylamine as chiral pool to make key intermediate (S)-3-(1-(dimethylamino)ethyl)phenol with multi-step reactions; then condense it with ethyl(methyl)carbamic chloride before finally salifying it with L-(+)-tartaric acid to receive rivastigmine. Although the optical purity of the starting material is guaranteed, but they all require selective introduction of a functional group at the meta position of S-phenylethylamine, and the results are basically mixtures, which are difficult to purify.
The methods listed above either have a long resolution process with low yield, or difficulty to purify intermediates, which make them unsuitable for industrial production; plus the purity of the rivastigmine resulted cannot be guaranteed.